1. Field of the Invention
The present invention relates to a wide-angle lens suitable for a single-lens reflex camera and a digital camera, an optical apparatus using the wide-angle lens, and a method for focusing the wide-angle lens.
2. Related Background Art
There has been proposed a retro-focus lens, in which a negative lens group comes to the most object side thereof, as a wide-angle lens securing a sufficient back focal length suitable for a single-lens reflex camera or a digital camera in spite of a short focal length thereof. In this type of a wide-angle lens, there has been proposed one having a large aperture ratio with an f-number of about 1.4 (for example, Japanese Patent Application Laid-Open No. 63-061213).
However, since power distribution of a retro-focus lens is asymmetric about an aperture stop thereof, astigmatism and coma tends to vary severely upon taking a picture of a close object in comparison with a picture of an object locating at infinity, so that it is difficult to construct a retro-focus lens to be a large aperture ratio. Moreover, since power distribution of a retro-focus lens is asymmetric such that the object side thereof is a negative lens group and the image side thereof is a positive lens group, it becomes extremely difficult to correct negative distortion and coma.